3. Activity detection with simple presence detectors

3.1 Presence detector principle

Presence detectors based on infrared radiation measurement (known as PIR "Presence Infra Red sensors") were originally used as intrusion detectors in alarm systems to protect property. The passive sensing element detects variations in energy received in infrared (IR) wavelengths, such as those emitted by the body's surface. To detect movement, a Fresnel lens is added to the radiation path to produce multiple paths for the light waves and thus create a differential signal when the moving subject crosses two successive paths (figure 1 ).